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 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.util;

/**
 * This class represents an observable object, or "data"
 * in the model-view paradigm. It can be subclassed to represent an
 * object that the application wants to have observed.
 * <p>
 * An observable object can have one or more observers. An observer
 * may be any object that implements interface <tt>Observer</tt>. After an
 * observable instance changes, an application calling the
 * <code>Observable</code>'s <code>notifyObservers</code> method
 * causes all of its observers to be notified of the change by a call
 * to their <code>update</code> method.
 * <p>
 * The order in which notifications will be delivered is unspecified.
 * The default implementation provided in the Observable class will
 * notify Observers in the order in which they registered interest, but
 * subclasses may change this order, use no guaranteed order, deliver
 * notifications on separate threads, or may guarantee that their
 * subclass follows this order, as they choose.
 * <p>
 * Note that this notification mechanism has nothing to do with threads
 * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
 * mechanism of class <tt>Object</tt>.
 * <p>
 * When an observable object is newly created, its set of observers is
 * empty. Two observers are considered the same if and only if the
 * <tt>equals</tt> method returns true for them.
 *
 * @author Chris Warth
 * @see java.util.Observable#notifyObservers()
 * @see java.util.Observable#notifyObservers(java.lang.Object)
 * @see java.util.Observer
 * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
 * @since JDK1.0
 */
public class Observable {

  private boolean changed = false;
  private Vector<Observer> obs;

  /**
   * Construct an Observable with zero Observers.
   */

  public Observable() {
    obs = new Vector<>();
  }

  /**
   * Adds an observer to the set of observers for this object, provided
   * that it is not the same as some observer already in the set.
   * The order in which notifications will be delivered to multiple
   * observers is not specified. See the class comment.
   *
   * @param o an observer to be added.
   * @throws NullPointerException if the parameter o is null.
   */
  public synchronized void addObserver(Observer o) {
    if (o == null) {
      throw new NullPointerException();
    }
    if (!obs.contains(o)) {
      obs.addElement(o);
    }
  }

  /**
   * Deletes an observer from the set of observers of this object.
   * Passing <CODE>null</CODE> to this method will have no effect.
   *
   * @param o the observer to be deleted.
   */
  public synchronized void deleteObserver(Observer o) {
    obs.removeElement(o);
  }

  /**
   * If this object has changed, as indicated by the
   * <code>hasChanged</code> method, then notify all of its observers
   * and then call the <code>clearChanged</code> method to
   * indicate that this object has no longer changed.
   * <p>
   * Each observer has its <code>update</code> method called with two
   * arguments: this observable object and <code>null</code>. In other
   * words, this method is equivalent to:
   * <blockquote><tt>
   * notifyObservers(null)</tt></blockquote>
   *
   * @see java.util.Observable#clearChanged()
   * @see java.util.Observable#hasChanged()
   * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
   */
  public void notifyObservers() {
    notifyObservers(null);
  }

  /**
   * If this object has changed, as indicated by the
   * <code>hasChanged</code> method, then notify all of its observers
   * and then call the <code>clearChanged</code> method to indicate
   * that this object has no longer changed.
   * <p>
   * Each observer has its <code>update</code> method called with two
   * arguments: this observable object and the <code>arg</code> argument.
   *
   * @param arg any object.
   * @see java.util.Observable#clearChanged()
   * @see java.util.Observable#hasChanged()
   * @see java.util.Observer#update(java.util.Observable, java.lang.Object)
   */
  public void notifyObservers(Object arg) {
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
    Object[] arrLocal;

    synchronized (this) {
            /* We don't want the Observer doing callbacks into
             * arbitrary code while holding its own Monitor.
             * The code where we extract each Observable from
             * the Vector and store the state of the Observer
             * needs synchronization, but notifying observers
             * does not (should not).  The worst result of any
             * potential race-condition here is that:
             * 1) a newly-added Observer will miss a
             *   notification in progress
             * 2) a recently unregistered Observer will be
             *   wrongly notified when it doesn't care
             */
      if (!changed) {
        return;
      }
      arrLocal = obs.toArray();
      clearChanged();
    }

    for (int i = arrLocal.length - 1; i >= 0; i--) {
      ((Observer) arrLocal[i]).update(this, arg);
    }
  }

  /**
   * Clears the observer list so that this object no longer has any observers.
   */
  public synchronized void deleteObservers() {
    obs.removeAllElements();
  }

  /**
   * Marks this <tt>Observable</tt> object as having been changed; the
   * <tt>hasChanged</tt> method will now return <tt>true</tt>.
   */
  protected synchronized void setChanged() {
    changed = true;
  }

  /**
   * Indicates that this object has no longer changed, or that it has
   * already notified all of its observers of its most recent change,
   * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
   * This method is called automatically by the
   * <code>notifyObservers</code> methods.
   *
   * @see java.util.Observable#notifyObservers()
   * @see java.util.Observable#notifyObservers(java.lang.Object)
   */
  protected synchronized void clearChanged() {
    changed = false;
  }

  /**
   * Tests if this object has changed.
   *
   * @return <code>true</code> if and only if the <code>setChanged</code> method has been called
   * more recently than the <code>clearChanged</code> method on this object; <code>false</code>
   * otherwise.
   * @see java.util.Observable#clearChanged()
   * @see java.util.Observable#setChanged()
   */
  public synchronized boolean hasChanged() {
    return changed;
  }

  /**
   * Returns the number of observers of this <tt>Observable</tt> object.
   *
   * @return the number of observers of this object.
   */
  public synchronized int countObservers() {
    return obs.size();
  }
}
